Magnetic resonance imaging (“MRI”) systems generally use strong magnetic fields in order to polarize the magnetic spins of nuclei to be imaged and to create the magnetic resonance condition therein. The magnetic fields should be approximately uniform (i.e., homogeneous) in order to perform high quality magnetic resonance imaging or spectroscopy of the nuclei. While magnets used to produce these fields are specifically designed and manufactured to achieve fairly high levels of homogeneity, typically additional local supplemental magnetic fields (e.g., the “shim fields” or “shims”) are added to achieve the final desired level of magnetic field homogeneity. There is a need for these shims because of residual field variations resulting from the magnet's manufacturing, and because of the alterations of the magnetic field resulting from the presence within the magnet of the subject to be imaged. These subject-dependant effects are important at higher magnetic field strengths, which are increasingly being utilized. The shim fields vary in predetermined ways with position, depending on the construction of the coils that generate the shim fields, and their strength can be controlled, by changing the current that passes through the shim coils. While the shim coils are generally designed to have approximately polynomial dependence on position, this approximation may break down away from the isocenter of the magnet. In the process of shimming the magnetic field, a combination of shim field strengths is generally sought that will locally compensate for the residual magnetic field variations.
The shim fields can typically be generated in either of two ways. One way this can be accomplished is by using pieces of magnetizable metal placed appropriately around the region to be imaged (typically referred to as “passive shimming”). Another way this could be done is by using current flowing in sets of conducting elements (“shim coils”) incorporated in the magnet housing which have been designed to have different specific locally varying magnetic field patterns whose overall strength is proportional to the currents (typically referred to as “active shimming”). In the latter method, the currents flowing in the shim coils can be adjusted so that their combined magnetic fields at least partially balance out the residual areas of inhomogeneity of the main magnetic field.
The determination of which combinations of shim currents to use can be carried out as follows: (1) by interactively adjusting the different currents with a sample in place in the magnet, and analyzing the resulting effect on the magnetic resonance signal from the sample, traditionally performed in a magnetic resonance spectroscopy, and/or (2) by imaging the spatial distribution of the magnetic field within the sample with a suitably modified MRI method and seeking the best combination of shim fields to minimize the observed field inhomogeneity. The first approach is taken usually in traditional Nuclear Magnetic Resonance (“NMR”) spectroscopy systems. The second approach can be implemented when the MR system has imaging capabilities, whereby an image can be created of the magnetic field variation within the object being analyzed. Such imaged field data can be obtained as a 3-D image or as a set of 2-D images in different locations and orientations. In addition, NMR systems used primarily for spectroscopy can be used to generate such images using the linear shims as imaging gradients.
Thus, if a predetermined set of maps of the spatial variation of the shim fields is available, such maps can be used to calculate the optimal combination of shim fields to be used to compensate for the main field inhomogeneity. Previous methodologies for performing this calculation have focused on iterative or least squares type approaches to seeking an optimal combination of shim fields. The exemplary system and method according to the present invention is provided for calculating such combination of shim fields to correct for a magnetic field inhomogeneity but without the need for an iterative approach. The following references are incorporated herein by reference in their entirety: Van Zijl et al., “Optimized Shimming for High-Resolution NMR Using Three-Dimensional Image-Based Field Mapping,” Journal of Magnetic Resonance, Series A 111, 203–207 (1994); Hu et al., “A Fast, Reliable, Automatic Shimming Procedure Using 1H Chemical-Shift-Imaging Spectroscopy,” Journal of Magnetic Resonance, Series B 108, 213–219 (1995).